The Michelson interferometer employing a laser is widely utilized in the field that requires nanometer measurement. An interferometer of this type needs to receive reflected light from a measurement-target object. For this purpose, mirrors are generally used. In the method utilizing mirrors, since optical alignment must be adjusted with extremely high precision, normally a cube corner reflector is employed for the measurement target so that interference measurement is not interrupted by deviation of the optical alignment. However, because a cube corner reflector is a prism, it cannot be attached to a microscopic location. In other words, if the measurement-target object is microscopic, position measurement employing an interferometer cannot be performed.
In view of this, the applicant of this invention has proposed to focus light on a mirror serving as the measurement target as shown in FIG. 8 so as to prevent a disturbance of the interference state caused by deviation of the mirror alignment (Japanese Patent Application Laid-Open (KOKAI) No. 2001-076325).
Referring to FIG. 8, a divergent light beam outputted by a light source LD is converted to a moderately focused light beam BEAM, transmitted through a non-polarization beam splitter NBS, and separated into P polarization component and S polarization component by a polarization beam splitter PBS. More specifically, while the P polarization component is transmitted through the PBS and reflected by a reference mirror M2, the S polarization component is reflected by the PBS and reflected by a measurement target surface M1. Then these reflected light beams are combined at the polarization beam splitter PBS and reflected by the non-polarization beam splitter NBS. The light beam reflected by the non-polarization beam splitter NBS goes through a quarter wavelength plate QWP and converted to linearly polarized light, whose polarization orientation rotates in accordance with a variation of a phase difference that bases upon a variation of the path length difference in the separated two light beams. The linear polarized light is divided into four light beams by a division device GBS. Each of the beams is transmitted through four polarization devices PP1, PP2, PP3 and PP4 arranged in a way that each of the polarization orientations is shifted by 45° In this manner, the four light beams are converted to four signal beams, whose interference cycles have 90° phase difference to each other, and received by respective photoreceptive devices PD1 PD2, PD3 and PD4, then four periodic signals are outputted.
In this conventional example, since light is focused on the measurement target, even if an alignment deviation (angle deviation) is generated, the wave surface of the reflected light does not change. Note that the center of the reflected light (optical axis) deviates. Since the wave surface does not change, the interference state between the reflected light and the reference light is stable.
Since this method does not use a cube corner reflector, it can be used as a new compact displacement sensor that measures a microscopic out-of-plane displacement on a measurement-target surface, with the use of a semiconductor laser for an optical sensor head.
However, since this method focuses light on a measurement-target surface, the spatial resolution becomes extremely high. Therefore, if a horizontal deviation or the like occurs on the measurement target object when measuring an out-of-plane displacement of the measurement-target surface, the superficial shape component of the measurement-target object is also detected, and the measurement may become unstable. Therefore, depending on the application, it is better to have a low spatial resolution in the direction of horizontal deviation on the surface for realizing stable out-of-plane displacement measurement. For this reason, widening the light irradiating area on the measurement target (mirror) has been desired.
Furthermore, in the above-described method, measurement is performed by focusing light on the measurement target. However, if there is a large out-of-plane displacement, the light focusing conditions are not satisfied, impairing the function of stabilizing the interference state to deal with an alignment deviation. For this reason, in general the measurable area has been limited to several tens of μm. Therefore, there have been demands for a method that enables easy alignment (enlarged allowance for an angle deviation) and that enlarges the measurable area of an out-of-plane displacement on the order of millimeter.